1. Field of the Invention
Numerous areas of the electronics industry require electronic materials and devices with precisely specified electrical properties. Material composition and strain are important parameters in determining a materials electronic properties, whilst accurate positioning and magnitude of dopants and interfaces in a device is essential to obtain optimum performance. Achieving these specifications is non-trivial and monitoring of electrical and material information, as well as process parameters such as wafer temperature, during fabrication and non-destructive testing are desired to optin se material, device and system performance.
2. Discussion of Prior Art
The largest areas of electronic production centres on the use of semiconductors. Within the next ten years semiconductor processing concepts are likely to undergo a fundamental change. Increasing demands for reduced dimensions and increased complexity will lead to enormous costs for conventional fabrication plants, which will become viable only for high volume markets. Low volume application specific devices and ICs will require more flexible, lower cost manufacturing approaches. Single-wafer processors, which rely for flexibility and control of specification on intelligent in-situ optical sensors, are seen as central to this development Feedback control of primary process parameters will need to be implemented in response to desired wafer parameters which will therefore need to be measured rapidly in-situ. Optical techniques such as reflectance1 and Ellipsometry (including both single wavelength and Spectroscopic Ellipsometry2 (SE)) are being developed to satisfy this role. Though they provide detailed information on the thickness and composition of a sample they are insensitive to electrical properties and this represents a major limitation in their use. Techniques which can provide electrical information in-situ and real-time have not been available. PSE is such a technique. Standard in-situ optical diagnostics can also be limited by their sensitivity to system variations (polariser speed in SE and light intensity in reflectance) which introduce errors in the calculated compositions and thicknesses. Errors are greater when monitoring thin layers where it becomes difficult to distinguish between the effects of thickness and composition on measured spectra.
An unambiguous determination of composition using a compatible technique which is insensitive to system variations would also simplify the interpretation of measured spectra to give reliable thickness information.
Photo-modulated ellipsometry provides such a determination being insensitive to both system variations and epilayer thickness effects (which complicate the interpretation of reflectance and ellipsometry data) in the samples. In-situ real-time measurement of temperature using SE is also prone to errors introduced by system variations and the presence of reflecting layers in a sample can introduce large errors in radiation measurements such as pyrometry. PSE provides a robust and rapid alternative method of measuring temperature.
Photo-modulated ellipsometry is a relatively new characterisation technique based on the high precision and electric field sensitivity of a photomodulated measurement and the insensitivity to noise and drift provided by ellipsometry. Ellipsometry is based on the measurement of the change in phase and amplitude of a light beam reflected from the sample surface. The measured ellipsometry parameters, "psgr" (amplitude ratio) and xcex94 (phase), can be related to the effective dielectric constant ( less than {tilde over (∈)} greater than = less than ∈1 greater than +i. less than ∈2 greater than ) of the sample, often referred to as the pseudo-dielectric function for spectroscopic measurements. Complex analysis can provide the dielectric function of a single layer, {tilde over (∈)}=∈1+i.∈2, in a multilayer sample or of a surface layer, which is important for monitoring of dynamic processes such as epitaxial growth where the surface is changing. The dielectric function can provide information on material temperature, composition and strain, but little direct information relating to electric fields. Photomodulated spectroscopies, such as photoreflectance (PR), use a laser beam to modulate surface and interface electric fields. Derivative spectra, of the reflected amplitude only, are obtained due to a field induced change in the localised dielectric function of the sample. PSE uses laser modulation in the same way yielding derivative spectra of the SE parameters, "psgr" and xcex94, and so xcex94{tilde over (∈)}=xcex94∈1+i.xcex94∈2 from each layer being modulated.
The similarity in origin of photo-modulated ellipsometry and PR signals allows techniques for the analysis of PR spectra to be adapted for use on PSE spectra. These are based on Apnes""s third derivative approximation3 which allows the derivative spectra to be fitted to provide critical point (CP) transitions energies in the material band structure amplitudes and broadnesses, which can then be related to material composition, strain, temperature and material quality. Observation of Franz-Keldish oscillations in the spectra can also be used to evaluate electric fields.
Modulated ellipsometry measurements have been reported as early as the late 1960s4,5 on metal samples, and using an electrolyte for modulation. Measurements were also made on Ge6,7 but little has been reported in the literature until 1990 when photo-modulated spectroscopic ellipsometry was first reported8. Since then, several PSE measurements have been reported on III-V semiconductor structures using standard SE instruments9-14.
PSE spectra were achieved using two different techniques. Both techniques used the point-by-point wavelength scans of the standard automated SE instruments. Xiong et al11-13 measured SE parameters one wavelength at a time for two individual SE spectra, one with the laser on and one with the laser off. Dielectric function spectra were calculated from these parameters and spectra with the laser off subtracted from those with the laser on to give the xcex94{tilde over (∈)}. Both CP transition energies13,14 and electric field10-14 information were obtained from these spectra. Zettler et al8-10 and Vanderhaghen et al15 chopped the laser beam and used phase sensitive detection to record PSE parameters.
According to this invention, a method of monitoring material parameters, including electric field; composition; strain; temperature or surface topography, during processing of material, comprises the steps of modulating the internal electric field of the material monitoring the change spectra that said modulation induces, using a real-time ellipsometer, and relating said change spectra to any of said parameters. The processing referred to includes manufacture and other industrial processes intended to modify the properties of the material.
The real time ellipsometer may be of the single wavelength type or of the spectroscopic type.
The internal electric field of the sample may be modulated using an electron beam or a laser. In the latter case, the parameters may further include laser power and laser alignment.